Study On The Molecular Systematics Of The Family Palaemonidae Rafinesque,1815(Crustaeca, Decapoda, Caridea)

The four palaemonoid (sub)families Anchistioididae, Gnathophyllidae,Hymenoceridae, and Pontoniinae are similar in morphology and all living in marinehabitats. Their systematic relationships are controversial. In this study, we usedsequences from four genes, including one mitochondrial ribosomal gene (16S rRNA)and three nuclear genes (H3, Nak and Enolase) to explore the phylogeneticrelationships among these four taxa. Our tree based on43species belonging to28genera shows that Gnathophyllidae, Hymenoceridae are nested within Pontoniinae.This result is consistent with evidence from larval morphology. The definingcharacteristics of Gnathophyllidae and Hymenoceridae, vestigial or missing mandibleincisor and broadened third maxilliped, same or similar forms could be found inPontoniinae as well; vice versa, based on description of species in literatures,gnathophyllids and hymenocerids meet the mainly diagnostic characteristics ofPontoniinae. The peculiar forms of third maxilliped presenting in gnathophyllids andhymenocerids might be the result of adaptive evolution, as well as the particularfeatures present in pontoniines. Anchistioididae is remoted to Pontoniinae based onphylogenetic tree, consistenting with the distinct morphological difference inpleopods. The pontoniine genera analyzed (together with Gnathophyllidae andHymenoceridae) are separated into two clades. The members of Clade I exhibitprimordial characters similar to that of Palaemoninae might be direct descendants ofancestor of Pontoniinae; and members of Clade II are relatively specialized.The phylogenetic relationships and evolutionary processes within the subfamilyPontoniinae, a speciose group of caridean shrimp with diverse lifestyles associatedwith coral reefs, are an interesting subject of study. In the present work, twomitochondrial ribosomal genes (12S and16S rRNA) and two protein-coding nucleargenes (histone3(H3) and the sodium–potassium ATPase α-subunit (Nak)) wereemployed to construct a phylogenetic tree including42genera and101species ofIndo-Pacific pontoniine shrimps. Compared to previous studies, ten additional generawere shown to be monophyletic groups, and the genera Dactylonia and Periclimenaeus were shown to be paraphyletic. The shallow-water crinoid-associatedpontoniines were divided into several groups according to the phylogenetic tree.These groups were mostly consistent with the morphological analysis conducted byprevious works. The studied bivalve-associated taxa exhibited ancestries that weretraceable to different lineages, and two groups could be distinguished:Anchiopontonia+Conchodytes and Anchistus. A similar situation occurred in the seaurchin-associated pontoniines. It is logical to speculate that pontoniines sharing thesame hosts may present different evolutionary pathways resulting from parallelintrusions of their hosts by morphologically plastic ancestral groups. Two hypotheseswere generated to account for the unique features observed in pontoniines:1) variousspecial ecological niches have been the major selective force molding the current highdiversity of pontoniines, and2) The transition of symbiosis in Pontoniinae results inthe morphological plasticity of pontoniines. In other words, the phylogeny of thePontoniinae is affectted by their hosts.The present study utilized a mitochondria gene (16S rRNA) and a nuclear gene(H3) to explore the phylogenetic relationships of subfamilies Palaemoninae andPontoniinae. The inner group contains31genera and39species, with specie ofAlpheidae (Alpheus gracilipes) as out group. The phylogenetic trees were constructedusing different methods and softwares. The scheme of two subfamiles withinPalaemonidae was denied by all the trees. And most of the genera which present bothmorphological characteristics of Palaemoninae and Pontoniinae were unresolved bythe phylogenetic trees. This work is just a preliminary study, if more molecularmarker and taxa can be involved, more precious and comprehensive phylogeneticrelationships within Palaemonidae could be figured out in the future.